The present invention generally relates to an automatic filter tuning control system for tuning the characteristic frequency of a filter to a target frequency automatically, and more particularly relates to measures to be taken to cut down the power dissipation and increase the precision of the tuning control system. As used herein, the xe2x80x9ccharacteristic frequencyxe2x80x9d of a bandpass filter means the center frequency thereof, while the xe2x80x9ccharacteristic frequencyxe2x80x9d of a high- or low-pass filter (HPF or LPF) means the cutoff frequency thereof.
A gm-C filter is often included in an integrated circuit for use in telecommunications. This filter utilizes the mutual conductance (gm; also called xe2x80x9ctransconductancexe2x80x9d) and capacitance (C) associated with a transistor to get its frequency characteristics controlled variably in accordance with some physical quantity including voltage and current. Exemplary configurations for filters of this type are disclosed in Japanese Laid-Open Publication Nos. 7-297677 and 2000-101392, for example.
A known automatic filter tuning control system for use in the receiver section of a telecommunications unit tunes a given filter every time the unit receives data, thus dissipating power more than necessarily. As for a cell phone, in particular, its standby mode operation cannot last so long, because a cell phone is normally driven by battery. So when performing the tuning control operation, a cell phone should reduce the power dissipation as much as possible. also, a cell phone now needs a BPF with a bandwidth as narrow as about 5% with respect to the center frequency thereof. Thus, the BPF should have its center frequency controlled highly precisely (on the order of 0.2 to 0.3%) in a shortest possible time.
An automatic filter tuning control system for finely adjusting the dip frequency of a biquadratic (or shortly, biquad) filter, one of gm-C filters of various types, is disclosed in Japanese Laid-Open Publication No. 63-167511 (Japanese Publication for Opposition No. 7-120923). The tuning control system disclosed in this publication is supposed to be built in an integrated circuit as a multiplexed audio frequency demodulator for a TV receiver. The control system inputs a sine wave signal with a constant frequency to a filter to be tuned and controls the output of a digital-to-analog converter (DAC) using a microcomputer, thereby gradually changing the characteristic of the filter. In this control system, the output level of the filter reaches a predetermined reference level for two input values of the DAC. Accordingly, the average of these two DAC input values is regarded as an optimum tuning value and stored on a nonvolatile memory.
Japanese Laid-Open Publication No. 5-114836 discloses an automatic filter tuning control system for finely adjusting the characteristic frequency and quality (Q) factor of a gm-C filter. The Q factor of a filter represents the frequency selectivity thereof. This tuning control system inputs an impulse signal, pulse signal or step signal, not a sine wave signal, to a filter to be tuned. An oscillating waveform appearing at the filter""s output is converted by an analog-to-digital converter (ADC) into a digital quantity, which is input to a microcomputer and then subjected to a Laplace transform. In this manner, the microcomputer calculates the characteristic frequency and Q factor of the filter being tuned. After the characteristics of the filter being tuned have been detected in this manner, tuning values are determined for two DACs for use to control the characteristic frequency and Q factor, respectively, thereby adjusting differences between the detected characteristics and the target ones. Then, those tuning values are stored on a nonvolatile memory. And when the filter is operated, the filter will have its characteristics controlled using the tuning values stored on the nonvolatile memory.
A automatic filter tuning control system as disclosed in Japanese Laid-Open Publication No. 2000-59162 inputs an impulse signal or step signal to a filter to be tuned and measures some periods of an oscillating waveform appearing at the filter""s output. In accordance with the result of this measurement, the control system detects the characteristic frequency of the filter. And then the control system finely controls the characteristics of the filter in such a manner as to adjust the difference between the characteristic and target frequencies. It should be noted, however, that the control system of this type is supposed to tune the filter every time the supply voltage or temperature has changed.
The known tuning control system disclosed in Japanese Laid-Open Publication No. 63-167511 (Japanese Publication for Opposition No. 7-120923) looks for an optimum tuning value by changing the DAC input value little by little. Thus, it takes a long time to complete the filter tuning of this type. Particularly when this technique is applied to a cell phone, the filter tuning process dissipates too much power because the tuning process is carried out every time the phone sounds. Also, a high-precision analog level detector is hard to realize generally speaking. Accordingly, it is difficult for such a control system to attain that high frequency tuning precision of about 0.2 to 0.3% normally required for a cell phone.
In the conventional tuning control system disclosed in Japanese Laid-Open Publication No. 5-114836, the microcomputer takes charge of the Laplace transform to compute the characteristic frequency and Q factor of the filter being tuned. Thus, the microcomputer should carry too heavy a load.
According to the technique disclosed in Japanese Laid-Open Publication No. 2000-59162, every time the supply voltage or temperature has changed, the filter tuning must be performed, thus also dissipating too much power just for that purpose. Furthermore, if noise is superimposed on the oscillating waveform appearing at the filter""s output, then the period measured will have a significant error. In that case, the characteristic frequency cannot be adjusted so precisely.
It is therefore an object of the present invention to provide a high-precision automatic filter tuning control system that can exhibit so high noise immunity and can reduce its power dissipation so much as to be effectively applicable to the receiver section of a cell phone, for example.
To achieve this object, the control system of the present invention detects and adjusts the characteristic frequency of a filter being tuned using an impulse signal, pulse signal or step signal and then stores the tuning result on a memory for future reuse. In addition, according to the present invention, only while the filter is being tuned, the circuit configuration of the filter is replaced with an alternative configuration that realizes a signal-to-noise ratio (SNR) high enough to control the filter characteristics as intended.
Specifically, an automatic filter tuning control system according to the present invention is for use to tune a characteristic frequency of a filter to a target frequency. The system includes circuit configuration replacing means, characteristic tuner and controller. While the filter is being tuned, the replacing means replaces an original circuit configuration of the filter with an alternative tuning-dedicated circuit configuration. The filter with the alternative configuration has the same characteristic frequency as that of the filter with the original configuration and shows an SNR higher than that of the filter with the original configuration. The characteristic tuner measures one or some periods of an oscillating waveform appearing at the output of the filter with the alternative configuration when an impulse signal, pulse signal or step signal is input as a test signal to the filter. Next, the tuner detects the characteristic frequency of the filter in accordance with the period measured and then supplies a tuning signal to the filter, thereby adjusting a difference between the characteristic and target frequencies. The controller issues a tuning instruction to start the characteristic tuner and then stores a level of the tuning signal when the difference between the characteristic and target frequencies of the filter enters a tolerance range. In operating the filter, the controller restores the filter to the original configuration, stops operating the characteristic tuner and controls the characteristics of the filter using the tuning signal stored.
The circuit configuration replacing means may include means for boosting a gain of the filter being tuned, means for increasing a Q factor of the filter being tuned or means for oscillating the filter being tuned at the characteristic frequency of the filter.